Caption: Soil thermometer. Thermometer stuck in the ground to measure the temperature of the soil. The thermometer consists of a narrow tube connected to a reservoir containing mercury (red). The mercury expands as it warms, forcing the liquid level up the tube. The liquid level can then be compared with a marked scale to show the temperature. The temperature of the soil changes much more slowly than that of the air. Some plants are not adapted to live in a cold climate, therefore it is important to know the soil temperature before moving plants from a warm environment, such as a greenhouse, into the open air.